In general with regard to motorcycles an output from an engine is transmitted to a driving wheel via a clutch. An engine brake is applied when sudden downshifting is carried out or on a similar occasion, resulting in a decrease in a grip force from a road surface. The driving wheel may at times run idle due to a reduced tire grip force. To respond to such situations occurring as a result of the engine brake being applied, a control has been proposed for engaging and disengaging the clutch or for providing a back torque limiter. See, for example, Japanese Patent Laid-open No. 2003-294062.
Japanese Patent Laid-open No. 2003-294062 controls the clutch by estimating an engine brake operating condition based on a parameter indicating a vehicle operating condition. The vehicle operating condition can, however, be varied by application of the brake only to the driving wheel or changes in road surface conditions, other than the engine brake. As a result, the means of estimation using the parameter indicating the vehicle operating condition may not be able to identify correctly the tire grip condition.